SALT-MARSHES OF NORTHUMBERLAND AND DURHAM. 133 



slenderer form of the limbs. In general structure and conform- 

 ation of body, in the peculiar arrangement of setae, the serra- 

 tion of the abdominal segments, in short, in those points where 

 distinct species mostly diverge, there is here a striking agree- 

 ment. * * After diligent enquiry, these differences remained 

 unexplained ; and I was inclined to consider them as mere in- 

 dividual variations. But further investigation of all parts of 

 the body convinced me that two distinct forms, with qualities 

 diversely useful, had originated two separate races, one slen- 

 der, swift, and agile, the other clumsy in figure, but robust and 

 powerfully armed. The two races are so far separate that inter- 

 mediate individuals, partaking of the characters of both, are not 

 met with. The upper antennae, however, in each case, show a 

 tendency to similar variations : at the same time these variations 

 are not so profound that they might not have been acquired 

 singly, or in combination. The differences in the relative size 

 of the claws and prehensile organs may be traced back to the 

 youngest stages of growth. * * * Many species may, 

 no doubt, have been founded on characters no more distinct than 

 these, and on mere deviation of character in the joints, which a 

 critical investigation would prove to be worthless."* It may be 

 noticed that many of the peculiarities here pointed out by Dr. 

 Glaus — especially the strong, doubly-curved claws of the first 

 feet in the stronger, and the very slender, simply-curved claws 

 in the weaker, form — have their exact counterparts in the two 

 varieties of D. tisboides here referred to. 



Genus. DELAVALIA,f nov. gen. 



In general form like Dactylopus. Superior antennae eight - 

 jointed, having no flagellum. Inferior antenna bearing a bi- 

 artieulated secondary branch. First pair of feet two-branched, 



* " Die Copepoden-Faima von Nizza. Ein Beitrag zur Charakteristik der Formen und 

 tlcren Abanderungen. 'im sinne Danvin's'" von Dr. C. Claus. Marburg und Leipzig-, 

 1866. See also an Abstract of Dr. Claus's Memoir, by the present aiitlior, in Uie " Intellec- 

 tual Observer, Vol. X., p. 327. 



t From Scaton Dclaval, near ^-liiih pi icc tlie genus wa.s first found. 



